Process for operation of a data link

ABSTRACT

The invention relates to a process for the operation of a data link between a base station and one or several mobile stations. According to said process data packets are transmitted between the base station and the mobile stations within transmission phases. The start of each transmission phase is indicated each time through emission of a start signal, and the transmission interface between the base station and the mobile stations for the transmission phase in question is managed by the base station. The data packets are formed with the data of a data stream and a received data stream is formed with the received data packets. The start and the end of each data packet forming operation is each time triggered by the start signals.

CROSS-REFERENCE TO A RELATED APPLICATION

This application is a National Phase Patent Application of InternationalPatent Application Number PCT/EP2005/000975, filed on May 26, 2005,which claims priority of German Patent Application Number 10 2004 026487.2, filed on May 27, 2004.

BACKGROUND

A process for operation of a data link is for example known from WLAN(Wireless Local Area Network) systems. In this previously known process,a data link between a base station and one or several mobile stations isoperated in that data packets are transmitted between the base stationand the mobile stations within transmission phases. In WLAN systems, thetransmission phases can be constituted by “contention-free” periods. Thestart of each transmission phase is indicated each time by the emissionof a start signal, which clearly can also be described as a beaconsignal; after the emission of the beacon signal, the air interfacebetween the base station and the mobile stations for the transmissionphase in question is reserved or managed by the base station. Withineach transmission phase, the mobile stations are addressed by the basestation and called up for the exchange of data packets. Each time afterthe completion of each transmission phase, there follows a trans-missionpause, in which no transmission of data packets controlled or managed bythe base station takes place between the base station and the mobilestations. Since in these transmission pauses the air interface is notmanaged by the base station, any other devices can gain access to theair interface in these transmission pauses. Accordingly, in WLAN links,these transmission pauses are also described as “contention” periods. InWLAN networks, the sending of the beacon signals by the base stationtakes place at regular time intervals, for example every 10.24 msecs, sothat a new transmission phase is created every 10.24 msecs.

In the previously known WLAN process, the data packets are formed withdata of a data stream, in particular a speech and/or video data stream;a received data stream is then created with the received data packets.The access to the transmission medium (air interface) in the previouslyknown WLAN process is normally effected by the CSMA/CA process (carriersense multiple access with collision avoidance), wherein the individualmobile stations competitively access the air interface. Hence it cannotbe predicted when a data packet due for transmission can actually betransmitted.

The standard IEEE802.11e (QoS extensions for WLAN) now offers thepossibility of coordinating the access to the medium via the basestation (access point). For this, IEEE802.11e defines a functionality“Hybrid Coordination function (HCF)”, which in turn defines acoordination function for the access of the mobile stations to thetransmission medium called “HCF Controlled Channel Access (HCCA)”. Putsimply, in the process the individual mobile stations which have appliedfor such a transmission mode are successively interrogated by the basestation (Access Point) directly after the beacon emission and their datapackets are thus collected. The time conditions for this interrogationmechanism are defined such that a mobile station cannot of itself gaincontrol over the transmission medium.

The U.S. patent application No. 2004/0066783 A1 discloses a process forthe organisation of a network with a central structure. In order tomanage links with devices which operate with different transmissionprocesses and require different transmission quality levels, for eachlinkage scheme an identification signal for the link type in question isalso supplied. A linkage specification is assigned to each linkage type.

Compared to synchronous transmission processes (compare for example theknown DECT (Digital European Cordless Telephone) process), wherein thetransmission of a speech data packet always takes place at exactly thesame time point relative to the frame start, with the process describedat the outset and on which this invention is based the exact time of thetransmission within the transmission phase is not fixed. Changes in thetransmission times can for example arise through the arrival of furthermobile stations or the departure of mobile stations and in some casesthrough repetitions of unsuccessful transmission attempts; the changesin the transmission times can sometimes considerably influence thetransfer rate in the transfer of a data stream, as was found by theinventors.

A process for bandwidth assignment in IEEE802.11e WLAN networks, using afunction known as a hybrid coordination function (HCF) and a hybridcoordinator (HC) for access management is known from the publication, L.A. Grieco et al., “A Control Theoretic Approach for Supporting Qualityof Service in the IEEE802.11e WLANs with HCF”, Proceedings of the42^(nd) IEEE Conference on Decision and Control, December 2003. Thispublication investigates, in particular, how transmission opportunitieshave to be correctly distributed within a “contention-free” period,while allowing for the time requirements of traffic categories, forexample for audio and video applications. The aforementioned publicationdiscloses how an accumulation of signals in a mobile station can beprocessed for a number of traffic categories in the presence ofinterference, by allocating the bandwidth of the WLAN network in such away that the signal accumulation of each traffic category is processedwithin a contention-free period.

SUMMARY

The invention is therefore based on the object of further developing aprocess of the type stated at the outset, in such a manner that thedelay arising during the transfer of a data stream becomes as minimal aspossible.

According to the invention it is provided that the start and the end ofeach data packet forming operation is triggered each time by the beaconsignals.

A considerable advantage of the process according to the inventionconsists in the fact that the data streams are transmitted with minimaldelay, since the data stream is packed into packets, the packet lengthwhereof for example always corresponds to the time interval between twodirectly consecutive beacon signals or alternatively in some cases thetime interval between two beacon signals separated by one or severalfurther beacon signals. The data packet forming operation is thusindependent of the transmission time point of the data packets withinthe transmission phase in question. Hence, at the receiving end, directassembly of the data packets is possible, without having to take accountof their transfer period. Errors and consequent delays in the assemblyof the data packets received, which can arise with data packet formationthat is “variable” with time or undefined, are thus avoided.

The process is preferably carried out with real-time critical datastreams, especially for example with audio or video data streams.

Preferably, each transmission phase is followed each time by atransmission pause, in which no data packet transfer for useful datatransmission takes place.

Preferably the data packets are transmitted between the base station andthe mobile station each time in that transmission phase whose beaconsignal also triggers the end of the data packet forming operation inquestion. Thus, in other words, in this mode of operation, the speechdata packets are formed in such a manner that the most recent data ofthe data stream on the sender side (base station or mobile station) isalways assembled into a data packet at the beginning of the beaconemission and then sent.

Preferably the data packets received each time are temporarily storeduntil the end of a predefined storage period after the occurrence of thebeacon signal triggering the transmission phase in question, before thereceived data stream is formed with the received data packets. In thisway, the occurrence of a time jitter in the formation of the receiveddata stream is reduced. The predefined storage period preferably dependson the time period between two consecutive beacon signals. For examplethe predefined storage period is half the time period between twoconsecutive beacon signals. In the latter case, the length of thetransmission phases should preferably always be smaller than, or at themaximum as large as, half the time period between two consecutive beaconsignals, in order to ensure undistorted received data stream formation.

Alternatively, the received data packets can be temporarily stored untilthe end of the transmission phase in question (U), before the receiveddata stream is formed with the received data packets.

Preferably, the data packets received, i.e. at the receiving end, aretemporarily stored until the end of the transmission phase in question,before the received data stream is formed or “continued” with the datapacket received each time. This mode of operation ensures that all thedata of a data packet needed for formation of the received data streamare always transmitted: a lack of data from a data packet and consequentdefective received data stream formation are thus avoided.

Particularly preferably, the duration of the transmission phases eachtime is at most half of the time interval between two consecutive beaconsignals. In this way, the maximal delay that can arise in the formationof the received data stream is limited.

In an advantageous form of the process according to the invention, theaim is for the mobile stations to be able to effect seamless handoverprocedures to other base stations. Here a handover procedure isunderstood to mean that a mobile station changes its base station, i.e.switches from the original base station to another base station, forexample because the transmission quality (e.g. signal strength, signalto noise ratio, bit error rate, etc.) in relation to the original basestation has deteriorated. During the handover procedures, thetransmission quality should not, or at least not significantly, beimpaired, so that for the users of the link, for example for thetelephone users in the case of a telephone link, the handover procedureis as far as possible imperceptible. In the context of the advantageousform of the process, this is achieved in that each time after datapacket transmission has been effected each mobile station is omittedfrom the data packet transmission for at least one followingtransmission phase; as soon as the mobile stations wish to prepare for ahandover procedure, they switch into a monitoring phase outside thetransmission phases used for the data packet transmission with the basestation. In this monitoring phase, the radio traffic, in particular atother frequencies than the transmission frequency of the assigned basestation, is listened to, and another (new) base station suitable for thedata package transmission is sought. An advantage of this form of theprocess consists in that, during this, time windows are deliberatelycreated for the mobile stations, wherein the mobile stations can preparefor a handover procedure if required. This is achieved through the factthat each mobile station does not have to transmit data packets in everyone of the transmission phases “made available” by the base station, butinstead of this is regularly “released” for at least one transmissionphase. Through the deliberate omission of transmission phases, a freetime space is created, in which the mobile stations can monitor theradio traffic at other frequencies and can seek other base stationsbetter suited for the data transmission. The described advantageous formof the process is for example to be recommended with all real-timecritical data streams, in particular for example with audio (e.g. audiodata formed in accordance with the “DECT” standard) or video datastreams which are transmitted via WLAN, since a quasi interruption-freehandover procedure is enabled.

It is considered advantageous for the mobile stations, in the event ofthe availability of another suitable base station, to set up a parallellink with the other base station for the preparation of the handoverprocedure, during which time windows which lie outside the transmissionphases used for the data packet transfer with the original base stationare used for the parallel link. Through the formation of an interimparallel link, it is ensured that a loss of data packets during thehandover procedure is avoided.

In the case of parallel data packet transfer, it is consideredadvantageous for the mobile stations to create the data packets for theoriginal base station at the instigation of the beacon signals of theoriginal base station and the data packets for the other (new) basestation at the instigation of the beacon signals of the other (new) basestation. In this way, a best possible synchrony of the data streams atthe changeover location in question, and hence a “seamless handover” inthe changeover, are achieved. Asynchronies, which arise due to mutuallydivergent time-bases in the base stations involved (time mismatchbetween the beacon signals of the base stations), during the briefduration of a handover procedure cause only a small synchrony mismatchof the data streams of a few sample values at the changeover location inquestion, which is imperceptible, or only just perceptible.

Preferably the two base stations operate with different transmissionfrequencies. The beacon signals of the two base stations can be mutuallyasynchronous. Preferably, the beacon signals of both base stations aremade equidistant each time.

The process can for example be carried out according to the WLANstandard described at the outset; the base stations are accordingly eachconstituted by WLAN access points (AP'S). After the emission of thebeacon signals, the air interface for the frequency range in question isthus reserved each time with the creation of a “contention-free” period;between the transmission phases, the air interface in the frequencyrange in question is released for “contention” periods. The end of eachtransmission phase can for example be indicated each time by theemission of a “contention-free end signal” by the base station.

It is considered advantageous for the omission or the use of thetransmission phases to take place continuously such that each mobilestation performs a data packet transmission exclusively in every m-thtransmission phase, where “m” denotes a whole number greater than 1.

It is especially advantageous if each mobile station performs a datapacket transmission in every second transmission phase. In this lattercase, the data packets preferably have a data content whose duration ordata content corresponds to the data stream content during double thetime interval between two beacon signals; it thus remains ensured thatthe data stream is transmitted without data loss.

The time windows used for the parallel link preferably include thosetransmission phases of the original base station which are “omitted” inrelation to this base station.

After the link has been created with the other base station, theparallel link with the original base station is preferably ended, inorder to take the load off the air interface.

If several mobile stations are connected to the base station, it isconsidered advantageous for the assignment of the mobile stations to thetransmission phases which are used for the data packet transmission withthe base station in question to be effected evenly. For example, half ofthe mobile stations are enlisted for data packet transmission in all“odd” (first, third, fifth, etc.) transmission phases, and the otherhalf of the mobile stations in all “even” (second, fourth, sixth, etc.)transmission phases. Accordingly, each time the data packets should be“bundled” or formed such that, in spite of the use of only every secondtransmission phase, at the receiving end an uninterrupted, dataloss-free received data stream can be created; hence if only everysecond transmission phase is used, then the data packets must be twiceas large or contain twice as much useful data as would be necessary witha data packet transmission in every transmission phase.

Preferably, the time interval between two consecutive beacon signals isselected to be at least twice as large as the length of thecontention-free periods lying between them each time, if every “second”transmission phase is omitted by the mobile stations each time. In thiscase, the duration of the monitoring phase of the mobile stations ispreferably at least 1.5 times the time interval between two consecutivebeacon signals.

The time interval between two beacon signals can for example be between5 msecs and 15 msecs; with WLAN links an interval of 10.24 msecs is forexample selected. The beacon interval is thus relatively short inrelation to an audio link (as a rule an 8 kHz link).

The invention further relates to a base station for the operation of adata link with one or several mobile stations.

With respect to such a base station, the invention is based on theobject of making it possible for the delay arising in the transfer ofthe data stream to be as minimal as possible.

This object is achieved according to the invention by means of a basestation with a base station control device, which exchanges data packetswith the mobile stations within predefined transmission phases. The basestation control device each time indicates the start of eachtransmission phase by emission of a beacon signal and reserves the airinterface for the transmission phase in question. In addition, with thedata of a data stream, in particular of a speech and/or video datastream, it forms data packets, during which the start and the end ofeach data packet forming operation each time is triggered by the beaconsignals. Next the data packets are transmitted to the assigned mobilestation.

It is considered advantageous for the base station control device alsoto assign the transmission phases to the mobile stations in such amanner that each time after data packet transmission has been effected,each mobile station remains excluded from the data packet transmissionfor at least one subsequent transmission phase.

With regard to the advantages of the base station according to theinvention and with regard to further advantageous forms of the basestation according to the invention, reference is made to the aboveexplanations in connection with the process according to the invention.

The invention further relates to a mobile station for the operation of adata link with a base station.

With respect to such a mobile station, the invention is based on theobject of making it possible for the delay arising in the transfer ofthe data stream to be as minimal as possible.

This object is achieved according to the invention by means of a mobilestation with a mobile station control device, which exchanges datapackets with the base station within predefined transmission phases,forms data packets with the data of a data stream, in particular of aspeech and/or video data stream, during which the start and the end ofeach data packet forming operation each time is triggered by the beaconsignals, and transmits the data packets to base station.

It is considered advantageous for the base station control device alsoto omit at least one subsequent transmission phase for data packettransmission each time after data packet transmission has been effected.

With regard to the advantages of the mobile station according to theinvention and with regard to further advantageous forms of the mobilestation according to the invention, reference is made to the aboveexplanations in connection with the process according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further explained with reference to the figures.

FIG. 1 shows a network with eleven mobile stations according to theinvention and three base stations according to the invention, and theprocess according to the invention is illustrated on the basis of thenetwork.

FIG. 2 shows the course of transmission before a handover procedure.

FIG. 3 shows the course of transmission during the handover procedure.

FIG. 4 shows the course of transmission after the handover procedure.

FIG. 5 shows a transmission procedure in detail.

DETAILED DESCRIPTION

In FIG. 1, four mobile stations MS1 to MS4 are seen, which are in a WLANradio link W with an access point AP1. Correspondingly, three mobilestations MS5 to MS7 are in a WLAN radio link W with an access point AP2,and four mobile stations MS8 to MS11 with an access point AP3.

The WLAN radio links W can for example be effected in accordance withthe standard IEEE 802.11a, b or g with HCF-QoS extensions in accordancewith IEEE 802.11e. The radio transmission takes place for example in theMHz or GHz range.

The three access points AP1, AP2 and AP3 are each connected to a centralswitch ZS (switching device), which is connected to a public telephonenetwork PSTN and/or to the internet. The links between the centralswitch ZS and the access points AP1, AP2 and AP3 on the one hand and thelink between the central switch ZS and the public telephone network onthe other are each for example constituted by a synchronous interface SY(e.g. ISDN-SO or UpO interface) or a packet-oriented interface withappropriate QoS precautions. Data streams D for example in the kHz range(e.g. telephone link) are transmitted via these interfaces SY.

In order to achieve as small time delays and as slight a jitter aspossible in the conversion of the data streams D into radio signals forthe WLAN link path W, the data packets for the WLAN link path W arecreated with “beacon-triggering” both in the access points AP1 to AP3and also in the mobile stations MS1 to MS11. During this, the datastream D is subdivided into packets, the packet content whereof alwayscorresponds to the data content in one time window, the time windowduration whereof is equal to double the time interval between twodirectly consecutive beacon signals. Transmission of the data packetsbetween the access points AP1 to AP3 and the respective assigned mobilestations MS1 to MS11 therefore takes place each time only in everysecond transmission phase; every second transmission phase is thusomitted.

If for example the signal quality in the data link between the mobilestation MS2 and the access point AP1 decreases, then the mobile stationMS2 must seek another access point with better transmission quality andcreate a link with this. Since different frequencies are assigned todifferent access points, the mobile station MS2 must retune on a trialbasis to another frequency, wait for a beacon on this frequency and, ifone is found, record the associated signal quality, for example thesignal strength. By repetition of this “scan procedure” at differentfrequencies, a table of possible access points is built up, in orderthen to seek the optimal access point as the target of the handover.

A problem now is that the access points AP1 to AP3, in contrast forexample to DECT base stations, are not mutually synchronised. Thebeacons (short for beacon signals) of the different access points arethus in any time position relative to one another, although they eachdisplay the same beacon repeat rate or the same beacon interval BA (seeFIG. 2). For example, the transmission phases of the access points AP1to AP3 may overlap.

Since the mobile station MS2 cannot know the time shift of the beaconsignals, it must, with a beacon signal interval of for example 10.24msecs, listen at the new frequency in question for at least ca. 10 msecsin order to intercept a possible beacon signal. This could lead to aninterruption in the data stream, since in the period in which the mobilestation MS2 is tuned to another frequency no data can be transmitted tothe old, original access point AP1.

In order to prevent such an interruption of the data stream, each accesspoint AP1 to AP3 divides the transmission phases in such a manner thateach assigned mobile station omits at least one transmission phase eachtime after each utilised transmission phase. For example, each mobilestation sends and receives data packets only in every second period.

This is shown by way of example in FIG. 2, in which the time sequence ofthe data packet transmission between the access points AP1 to AP3 andthe mobile stations MS1 to MS11 is shown. In each case, a “A” symbolrepresents a transmission in the mobile station direction and the symbolrotated through 180° a transmission in the access point direction. Thebeacon signals are marked with the symbol B and have a beacon intervalBA of for example 10.24 msecs.

The transmission phase, or “contention-free period”, triggered by thebeacon signal B is marked in FIG. 2 by the symbol U. Each transmissionphase U is followed each time by a transmission pause (“contentionperiod”) F, in which the air path is released for the frequency range inquestion.

Since each mobile station MS1 to MS13 [sic] each uses only every secondtransmission phase, the quantity of data per transmission phase eachtime is doubled, compared to a “normal” transmission in everytransmission phase, in order to obtain the required mean data rate.

As can be seen in FIG. 2, the mobile stations assigned to each accesspoint are preferably evenly apportioned to the “even” and “odd” beaconsor transmission phases, in order to attain an even loading of thetransmission phases.

Since only every second transmission phase relative to the access pointAP1 is used, the mobile station MS2 has sufficient time between the datatransmissions to the assigned access point AP1 to retune to anotherfrequency, to seek a beacon there, and tune back to the old frequency ingood time. The central point is that the beacon period is still always10.24 msecs, although the interval between the transmission phasesactually used is doubled, compared to the “normal” use of alltransmission phases.

Preferably, in each case the time interval between two consecutivebeacon signals, here 10.24 msecs, is at least twice as large as thelength of the contention-free period U lying between them; this meansthat the transmission phases may last a maximum of 5.12 msecs each time.Correspondingly, the duration of the monitoring phase M of the mobilestation MS2 can be 1.5 times the time interval between two consecutivebeacon signals, i.e. ca. 15 msecs. Accordingly, in this monitoring phaseof 15 msecs at least one beacon on the new frequency must berecognisable, irrespective of how the beacons of the threeunsynchronised access points AP1 to AP3 are displaced relative to oneanother, because the beacon interval at all access points is 10.24 msecsin each case.

If, as already mentioned above in connection with FIG. 1, for examplethe signal quality in the data link between the mobile station MS2 andthe access point AP1 decreases, then the mobile station MS2 scans theair interface at different frequencies for available access points. Iffor example in the process it is established that the access point AP2is suitable for a handover procedure, then the mobile station MS2 willcreate a parallel data link with the new access point AP2. This is shownin detail in FIG. 3.

As can be seen in FIG. 3, the assignment to the “even” or “odd” beaconat the new frequency of the new access point AP2 is selected in such amanner that in fact two parallel data streams are possible; this meansfor example that the mobile station MS2 must select an “odd”transmission phase in relation to the new access point AP2, if it is inan “even” transmission phase in relation to the old, original accesspoint AP1. In the handover phase, the mobile station MS2 on averagetransmits data every 10.24 msecs, which is alternately directed to theold and the new access point.

As soon as the creation of the parallel data link is completed, the linkto the original access point AP1 is broken off; this is shown in FIG. 4.

For better understanding, in FIG. 5 the data link between the accesspoint AP1 and the three mobile stations MS1 to MS3 in the “first”transmission phase according to FIG. 2 is shown once again. It can beseen that the access point AP1 firstly passes data packets “DATA” to themobile station MS1. As soon as this process is completed, data packets“DATA” are requested from the mobile station MS1 by means of a signalCF-Poll. Next, this process of the sending and “requesting” of datapackets is repeated with the mobile stations MS2 and MS3. The“contention-free periods” can for example be ended by a contention-freeend signal CF-end.

The data packets “DATA” always contain a data content which correspondsin time to double the time interval BA between two directly consecutivebeacons signals. The data packets “DATA” thus contain data of the datastream D for a time period of 2*BA.

1. A method for operating a data link between a base station and one ormore mobile stations, the method comprising: transmitting data packetsbetween the base station and the one or more mobile stations withintransmission phases; generating a start signal to indicate start of eachtransmission phase; managing a transmission interface between the basestation and the one or more mobile stations for a respectivetransmission phase by the base station, responsive to the start signal;transmitting data packets formed with data of a data stream to the oneor more mobile stations; receiving the data packets by the one or moremobile stations; and forming a received data stream with the receiveddata packets, wherein start and end of each data packet are triggered bya respective start signal; and wherein the received data packets aretemporarily stored until the end of the transmission phase, and beforethe received data stream is formed with the received data packets.
 2. Amethod for operating a data link between a base station and one or moremobile stations, the method comprising: transmitting data packetsbetween the base station and the one or more mobile stations withintransmission phases; generating a start signal to indicate start of eachtransmission phase; managing a transmission interface between the basestation and the one or more mobile stations for a respectivetransmission phase by the base station, responsive to the start signal;transmitting data packets formed with data of a data stream to the oneor more mobile stations; receiving the data packets by the one or moremobile stations; and forming a received data stream with the receiveddata packets, wherein start and end of each data packet are triggered bya respective start signal, wherein the start signal is a beacon signal,and wherein the received data packets are temporarily stored until theend of a predefined storage period after the occurrence of the beaconsignal triggering the transmission phase, and before the received datastream is formed with the received data packets.
 3. The method accordingto claim 2, wherein the predefined storage period depends on the timeperiod between two consecutive beacon signals.
 4. The method accordingto claim 3, wherein the predefined storage period corresponds to half ofthe time period between two consecutive beacon signals.
 5. The methodaccording to claim 2, wherein the data packets are formed with real-timecritical data streams, and are transmitted in the course of data packettransmission.
 6. The method according to claim 2, wherein the datapackets are formed with real-time critical audio or video data streamsand are transmitted in the course of data packet transmission.
 7. Themethod according to claim 2, wherein the data packets have a datacontent duration of which corresponds to an integer multiple of the timeinterval between two beacon signals.
 8. The method according to claim 2,wherein start of a contention-free period is indicated by the beaconsignals.
 9. The method according to claim 2, wherein the transmissioninterface is configured as a wireless interface, and wherein between thetransmission phases the wireless interface in a respective frequencyrange is released for contention periods.
 10. The method according toclaim 2, wherein the data packet transmission takes place in a WirelessLocal Area Network (WLAN) standard and the base station is constitutedby a WLAN access point.
 11. The method according to claim 2, wherein arange of the time interval between two beacon signals is between 5 msecsand 15 msecs.
 12. The method according to claim 2, wherein each timeafter data packet transmission has been effected each mobile station isomitted from the data packet transmission for at least one subsequenttransmission phase.
 13. The method according to claim 12, wherein eachmobile station is authorised for the data packet transmissionexclusively in every m-th transmission phase, where m denotes an integernumber greater than one.
 14. The method according to claim 12, whereineach mobile station is authorised for the data packet transmissionexclusively in every second transmission phase.
 15. The method accordingto claim 2, wherein outside the transmission phases used for the datapacket transmission with the base station, the mobile stations switchinto a monitoring phase, wherein radio traffic is listened to.
 16. Themethod according to claim 15, wherein in the monitoring phases, anotherbase station suitable for the data packet transmission is sought. 17.The method according to claim 16, wherein in the event of availabilityof a suitable other base station, the mobile stations set up a parallellink with the other base station, and wherein time windows which lieoutside the transmission phases used for the data packet transfer withthe base station are used for the parallel link.
 18. The methodaccording to claim 17, wherein the two base stations operate withdifferent transmission frequencies and the beacon signals of both basestations are mutually asynchronous.
 19. The method according to claim17, wherein the beacon signals of the two base stations are madeequidistant each time.
 20. The method according to claim 17, wherein themobile stations create the data packets for the base station at theinstigation of the beacon signals of the base station and the datapackets for the suitable other base station at the instigation of thebeacon signals of the suitable other base station.
 21. The methodaccording to claim 17, wherein the time windows used for the parallellink include the transmission phases omitted in relation to the basestation.
 22. The method according to claim 17, wherein after linkcreation with the suitable other base station has been effected, theparallel link with the base station is ended.
 23. The method accordingto claim 2, wherein the assignment of the mobile stations to thetransmission phases which are used for the data packet transmission withthe base station is effected evenly.
 24. The method according to claim2, wherein the time interval between two consecutive start signals isselected to be at least twice as large as the length of thecontention-free periods lying between the two consecutive start signals.25. The method according to claim 2, wherein the duration of themonitoring phase of the mobile stations is at least 1.5 times the timeinterval between two consecutive start signals.